In recent years, there have been increased needs toward constructing intranet corporate networks incorporating access to the Internet and the Internet Protocol (IP). Communication services provided by communication operating companies featuring provision of an end user interface through IP packets has started, in which a variety of methods are employed.
One form of such network configurations is a ring-structured layer 2 Ethernet network (the Ethernet is standardized in the IEEE 802.3.)
In FIG. 1, one exemplary configuration of such a layer 2 Ethernet network is shown. The network accommodated in a backbone network 100 is constituted of layer 2 (L2) switches (or bridges) #1-#8 connected in a ring shape.
Further, the Spanning Tree Protocol (STP) is known as a network configuration protocol. The STP is a protocol for logically making a tree-structured network, so as not to produce a loop structure in a layer 2 network (where layer 2 denotes the second layer in the Open System Interconnection, OSI. As for the STP, for example, refer to ANSI/IEEE Standard 802.1d.
Among switches (bridges) constituting a system, a representative bridge (which is referred to as root bridge) is selected, and a tree-structured network is logically configured from the root bridge. Also in the ring network shown in FIG. 1, it is possible to configure a tree-structured network logically.
The STP enables to configure logical links in FIG. 1, which include a first link directing in the clockwise direction from L2 switch #1 to L2 switch #5, and a second link directing in the counterclockwise direction from L2 switch #1 to L2 switch #6.
To enable such a tree structure by use of the STP, each switch (bridge) transmits a control frame (configuration frame) in an initialization process which is performed in the event of the power switched on. In a bridge receiving the control frame (configuration frame), when an operation system has already been established, the current root bridge (L2 switch #2) is remained as the root bridge, and the added bridge is newly incorporated in the system. Thus system reconfiguration is carried out.
A link weight between bridges is defined as a path cost. After the path cost is added to the value set in the control frame (configuration frame) which is transmitted from the root bridge, the frame is forwarded to a neighboring bridge.
In each switch (bridge), when control frames (configuration frames) are received (that is, two control frames are received totally) from the two ports connected to the ring, the switch (bridge) of interest compares the path costs which are set in the two control frames (configuration frames). As a result of this comparison, the port side which has received the control frame (configuration frame) having a greater path cost is treated as a blocking port, in which entire frames for transmission are blocked.
In the example shown in FIG. 1, the port provided in L2 switch #5 and is directing to L2 switch #6 is regarded as the blocking port. In such a way, a logical tree structure is formed.
Here, in a network set by the STP as described above, when a failure occurs on any link, for example, on a link located between L2 switch #7 and L2 switch #8, data transmission on a link directed from L2 switch #7 toward L2 switch #6 becomes unable. In such a case, it is necessary to remove the block condition having been set in the blocking port of L2 switch #5, as well as to perform a ring switchover so that a link directed from L2 switch #5 toward L2 switches #6 and #7 is established.
In a conventional ring-structured network, the STP (Spanning Tree Protocol) specified in the IEEE 802.1d is usually employed as a method for ring switchover in the event of a failure.
FIG. 2 shows a diagram illustrating STP state transitions in each port of a switch.
In FIG. 2, when the STP is enabled, there exist four states, A, B, C and D. The contents of each state are illustrated below.
(A) Blocking: A state in which a certain port does not take part in forwarding a control frame (configuration frame). In this state, a received control frame (configuration frame) is discarded without forwarding.
(B) Listening: A state in which a control frame (configuration frame) is ready for forwarding. Forwarding the control frame (configuration frame) is temporarily set disabled to prevent from forming a temporary loop.
(C) Learning: A state in which a control frame (configuration frame) is ready for forwarding. In this state, only receiving function is enabled, and a source Media Access Control (MAC) address is learned and registered into a table.
(D) Forwarding: A state in which a control frame (configuration frame) can be forwarded. In this state, a received control frame (configuration frame) is forwarded to a neighboring port.
Among the above-mentioned four states, the state is either (A) Blocking or (D) Forwarding during a stationary state.
Further, in FIG. 2, reference numbers (1)-(5) denote transition conditions, the contents of which are as illustrated below.
(1) A port state is changed from an STP disabled state to an STP enabled state, initiated either by an administrator or in the initial setting.
(2) A port is changed from the STP enabled state to the STP disabled state.
(3) Selection of a root port.
(4) Selection of an alternative port.
(5) Completion of a protocol timer (forwarding time)
Here, in FIG. 2, a port is in (A) Blocking state in the event of a failure. In order to reestablish a link by shifting to (D) Forwarding state, it is necessary to shift from (A) Blocking→(B) Listening→(C) Leaning→(D) Forwarding in the transition diagram shown in FIG. 2.
In order to shift from (A) Blocking→(B) Listening, in other words to perform (3) selection of a root port, a default time of 20 seconds is being set. Further, in order to shift from (B) Listening→(C) Learning, and from (C) Learning→(D) Forwarding, in other words as a transfer time, a default time of 15 seconds is being set, respectively.
Thus, according to the default setting, it is required 50 seconds for a link switchover by use of the STP to shift from (A) Blocking state to (D) Forwarding state in the event of a failure.
As described above, the STP is a protocol for avoiding occurrence of loop path in an L2 network, by which a time of 50 seconds is normally required for a system reconfiguration. This impedes to maintain a session of an application.